The present invention relates to a valve assembly wherein hydraulic pressure is used to provide the return force of a cam actuated valve. The inventive assembly is especially suitable for automotive use.
Cam operated engine valves are required to operate over a large RPM range, generally about 500 RPM to 5000 RPM. When using steel valves, the spring forces required for the valve gear to follow the cam over the required RPM range are about 100 to 300 lbs. The higher the RPM, or the more extreme the cam profile, the greater is the force required. An "extreme" profile, as the term is used herein, is one which causes sudden accelerations or decelerations of the valve during its reciprocatory motion.
Coil springs that are used in conventional valve assemblies have a complex distribution of mass and spring constants which cause them to start acting like poorly terminated transmission lines at the design limits, e.g. high RPM or with extreme cam profiles. Internal flexures and attendant complex resonances take place within the spring, causing internal surfaces of deflection which greatly affect the overall behavior of the spring and other valve gear. The behavior of the spring, in the extreme, could be analogized to that of a "Slinky" toy.
Valve seating velocity should increase linearly with engine speed. Due to the above described surging effects, however, there can be significant variations above and below the linearly determined seating velocity. When the valve no longer follows the cam, it is said to float, bouncing off the cam and against the valve seat. The resulting hammering of the cam and the valve seat can cause damage by cold working the surfaces, as well as wear resulting from removal of lubricants under the impact loading. Cold working of the exhaust valve and associated seat is exacerbated by the associated high temperatures.
A related problem is the scoring of the valve and valve seat due to the high gas pressure and velocity at the seat. As the seating deteriorates due to cold working and wear, the scoring is exacerbated. Poor seating and scoring cause the valve to get even hotter due to poor heat conduction to the seat.
In addition to the cold-working, wear, and scoring problems, valve float also causes inefficient engine operation due to randomness in the timing of the valve closure. This randomness can result in a loss of compression and a loss of expansion energy with attendant loss of engine power.
U.S. Pat. Nos. 3,120,221, 3,722,483, and 4,592,313 address the float problem by offering a pneumatic approach that requires a central air compressor. Due to the high compressibility of air, i.e., its low bulk modulus of elasticity, greater working displacements and more massive pistons are required.